Jan Esper of Johannes Gutenberg University in Mainz, Germany, thinks that at least some of those tree rings actually show something else&colon; a long-term cooling trend that lasted right up until the Industrial Revolution. The trend came about because of reduced solar heating caused by changes to the Earth’s orbit known as Milankovitch wobbles, says Esper. His results suggest the Roman world was 0.6 °C warmer than previously thought – enough to make grape vines in northern England a possibility.

Esper and his colleagues say that warmer summers do not necessarily make tree rings wider – but they often make them denser. He studied the density of tree rings in hundreds of northern Scandinavian trees and found that they showed evidence of a gradual cooling trend that began around 2000 years ago.

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The finding fits with other proxies for temperature – such as the chemical make-up of air trapped in glaciers and the organic remains in ancient lake sediments – which have also suggested a cooling trend.

Esper’s study is the latest to indicate that temperatures were less stable than originally thought. In 2009, Darrell Kaufman of Northern Arizona University at Flagstaff published evidence, using a range of proxies, that indicated a cooling in the Arctic for most of the past 2000 years (Science, DOI&colon; 10.1126/science.1173983). Esper’s findings suggest that the cooling trend was even stronger than Kaufman concluded.

The finding does not change our understanding of the warming power of carbon dioxide. In fact, it shows that human CO2 emissions have interrupted a long cooling period that would ultimately have delivered the next ice age.

Esper says temperature reconstructions will have to be redone because past studies probably underestimated temperatures during the medieval warm period and other warm periods going back to Roman times. The further back in time, the greater the underestimate would be.

But others have doubts. Mann argues that Esper’s tree-ring measurements come from high latitudes and reflect only summer temperatures. “The implications of this study are vastly overstated by the authors,” he says.